1. Field of the Invention
The present invention relates to a technique of blow moulding a hollow article and more specifically to a technique of blow moulding a hollow article which has a structure formed integrally therewith.
2. Description of the Prior Art
FIGS. 1 and 2 show a moulding technique disclosed in JP-A-55-20157. This technique is used for producing plastic fuel tanks and the like type vessels wherein it is necessary to provide one or more baffles in order to increase the structural rigidity of the article and/or to prevent excessive and/or undesired movement of the fuel contained therein when (for example) a vehicle, in which the tank is disposed, is subject to an acceleration or change in attitude.
In this prior art technique a parison 1 of thermoplastic resin such as polyethylene, polypropylene, polycarbonate, vinyl chloride or ABS resin is extruded in a molten plastic state through an essentially annular orifice A defined between a core 2 and a die 4. The parison is, as shown in FIG. 2, pressed into shape using a two piece water cooled mould 6. One or both of the dies 8, 9 of the mould are formed with projections 10 (only one shown in FIG. 2) which form an inwardly projecting deformation 12. This deformation 12 acts as a baffle in the finished article.
However, this prior art technique has suffered from the drawback that, as the baffle is formed by deforming the parison in the above mentioned manner, the elongation ratio of the some portions of the parison are much higher than the elongation ratio for the rest of the same with the result that it is difficult to stably control the wall thickness of the final product. Further, as the indent is relatively thick it tends to undesirably reduce the internal volume of the product.
A further drawback comes in that the formation of the baffles using the above disclosed technique tends to wrinkle the external surface of the product and reduce the aesthetic value of the same.
Moreover, the prior art technique is limited to the formation of baffles and the like protrusions and does not lend itself to the construction of a vessel which houses and/or includes more complex arrangements including tubing, measuring instruments or the like.